(b Hanau, Electoral Hesse, 30 October 1817; d Heidelberg, Germany, 20 February 1892)
Kopp’s father, Johann Heinrich Kopp, was a practicing physician who had a strongo interest in science. He taught chemistry, physics, and natural history in the local lyceum and possessed an outstanding mineralogical collection. He occasionally published papers on mineralogy and physiological chemistry. His son was thus exposed early in life to chemistry and crystallography, which later were his major scientific concerns. During his studies at the Hanau Gymnasium, however, Kopp’s subjects were chiefly Latin and Greek. When he entered the University of Heidelberg in 1836, he intended to study philology. His interest in chemistry was aroused by the lectures of Leopold Gmelin, and he therefore decided to devote himself to this subject.
Because there was little opportunity then for individual experimental work at Heidelberg, in 1837 kopp went to Marburg. He received his doctorate on 31 October 1838 with the dissertation De oxydorum densitatis calculo reperiendae modo, which revealed his early interest in the physical properties of substances. After a short period at Hanau, Kopp moved to Giessen in 1839 to work with Liebig, remaining there for twenty-four years. he became a privatdocent in 1841, lecturing on theoretical chemistry, crystalography, meterologyo, and physical geographyp. In 1843 he was appoionted extraordinary profoessor; and in 1852, when Liebig left Giessen for Munich, Kopp and Heinrich Will were jointly appointed to succeed him. Kopp did not like administrative work, and after a year he turned control of the laboratory over to Will. His relations with Liebig remained close, and he corresponded with him for the rest of his life. Most of Kopp’s experimental work was materials on the history of chemistry while there. He taught this subject at intervals early in his career, and in later life he made this one of his main teaching activities.
Kopp’s wife, Johanna, whom he married in 1852, came from Bremen. They had two sons, who died in infancy, and a daughter, therese. Both Kopp and his wife suffered from poor health much of their lives, and his letters to Liebig are full of complaints concerning illnesses.
In 1863 Kopp left Giessen for Heidelberg, where he spent the rest of his life. He was called three times to Berlin but always preferred to remain at Heidelberg. During his years there he gave courses only in crystallography and the history of chemistry. He retired in 1890 and died two years later.
As his dissertation had shown on, Kopp’s chief interest lay in the study of the physical properties of substances. Under the influence of Liebig when he first came to Giessen, Kopp studied the action of nitric acid on mercaptans; but this was his only excursion into the customary organic chemistry of his day. His real concern was an attempt to establish a connection between the physical properties and the chemical nature of substances. Since he seldom worked with students, he had to carry out most of his experiments by himself. Kopp’s first published paper (1837) had concerned the construction of a differential barometer; he delighted in designing and building the apparatus needed for his many accurate determinations of physical constants. His work involved many tedious purifications and much laborious calculation, But he enjoyed this type of study. In the course of his work he accurately measured the boiling points of many organic substances for the first time.
Beginning in 1839, Kopp studied the specific gravity of a number of compounds. In developing formulas for calculating such values he used the concept of specific volume, which he defined as the molecular weight divided by the specific gravity. He showed the similarity of this value in similar elements and isomorphous compounds and related it to their crystal structures, although he was unable to generalize the work to the extent he desired. In 1841 he observed the relations between chain length and boiling point in various classes of organic compounds. He pointed out the generally constant increase in this value as the chain length in a homologous series is increased by addition of a methylene group, but he stressed that the exact value for the increase varies in different types of compounds. He concluded that the boiling point of a liquid was a function partly of molecular weight and partly of chemical constitution.
In 1864 Kopp undertook the study of specific heats of a large number of elements and compounds, in an attempt to verify Neumann’s law that the product of molecular weight and specific heat is a constant, regardless of the nature of the substance. He found that in fact the relation was much more complicated and involved a large number of factors. He was, however, able to show that each element has the same specific heat in its free solid state as in its solid compounds. The specific heats of compounds could be calculated from those of their elements.
Kopp’s general conclusions often had to be modified later, but in many cases the evidence he presented of a relation between physical properties and chemical structure opened the way for advances in both organic and physical chemistry.
Many of Kopp’s researches were paralleled to some extent by the work of H. G. F. Schröder. Heated disputes between the two chemists frequently took place, and a considerable polemical literature resulted as Kopp asserted his priority in certain discoveries or the correctness of his interpretations. Although essentially mild-mannered, Kopp never hesitated to express his views when he felt aggreived.
While carrying on his laboratory studies, Kopp was also engaged in literary activities. With Liebig he continued publication, of the Jahresbericht über die Fortschritte der Chemie after the death of Berzelius, its Founder. As editors they changed the plan of the publication, making it a general review of chemistry and related subjects, and enlisted the aid of their university colleagues, so that amost all the membes of the philosophical faculty took part in the work. Kopp handled the details of general editorial management. He was also an editor for many years of Justus Liebigs Annalen der Chemie. As a result of teaching courses in crystallography he was able to prepare a well-known text on the subject, although he did little laboratory work in this field. It was through his historical books that he established his greatest reputation.
Kopp’s linguistic training was undoubtedly of great value to him in this work. He gave his first course of lectures on the history of chemistry when he was only twenty-frou; two years later he published the first volume of Geschichte der Chemie, which appeared in four volumes between 1843 and 1847. It is clear that he must have been collecting materials for a longer period than that required for giving his course. The first complete, accurate, and readable history of chemistry, the book was notable for its success in relating the development of chemistry to contemporary cultural events.
The first volume contained a general history of the science; the second consisted of individual histories of special branches of chemistry; the last two gave histories of individual substances, elements, and organic compounds. The style was simple and direct, in contrast with the very involved sentences characteristic of Kopp’s later historical writings. Ruska has suggested that this complex style came from continued reading of Latin authors. Since Kopp was writing in relative isolation from large libraries, he was not able in his frist historical work to utilize source material from early Greek and Arabic authors. The other major contemporary historians of chemistry, Ferdinand Hoefer and Marcellin Berthelot, working in Paris, were able to produce more complete surveys of these periods; but Ruska believes Kopp’s treatment of later eras was superior to theirs. His later works rectified the lack of Greek and Arabic material.
Shortly after his call to Heidelberg, Kopp was asked to prepare a history of recent chemical developments in Germany. The resulting Die Entwicklung der Chemie in der neueren Zeit (1873) went far beyond the original plan. It discussed the development of chemistry to about 1858, not only in Germany but in all the major countries. An internationalist in outlook, Kopp differed from the French historians, who tended to regard chemistry as a French science. As a result of this approach, Kopp was led in the later years of his life into polemical disputes with Berthelot. The latter very grudgingly mentioned the publications of Kopp and Hoefer on alchemy but remarked that he had reconstituted the whole science which others had neglected. Once more Kopp’s priority was challenged, and he responded with a strong defense of his work.
Kopp always intended to revise his Geschichte der Chemie, and during most of his life he collected materials for this purpose; but he was never able to put these in final form, and only his scattered publications on alchemy reveal the richness of his historical thought. After his death his material for revision of his Geschichte was lost, and so the four volumes of his history remain his chief monument.
I. Original Works. Kopp’s summary of his lifetime of experimental work is “Ueber die Molecularvolume von Flüssigkeiten,” in Justus Liebigs Annalem der Chemie, 250 (1889), 1-117. His cheif historical works are Geschichte der Chemie, 4 vols. (Brunswick, 1843-1847); Beiträge zur Geschichte der Chemie, 3 pts. (Brunswick, 1869-1875); Die Entwicklung der Chemie in der neueren Zeit (Munich, 1873); and Die Alchemie in alterer und neuerer Zeit (Heidelberg, 1886). there is a bibliography of his scientific papers by T. E. Thorpe, in Journal of the Chemical Society, 63 (1893), 782-785.
II. Secondary Literature. An appreciative obituary is by A. W. von Hofmann, in Berichte der Deutschen chemischen Gesellschaft, 25 (1892), 505-521. T. E. Thorpe, “Kopp Memorial LEcture,” in Journal of the Chemical Society, 63 (1893), 775-815, devotes particular attention to Kopp’s experimental work. Julius Ruska, “Hermann Kopp, Historian of Chemistry,” in Journal of Chemical Education, 14 (1937), 3-12, critially evaluates the historical writngs. Max Speter, “‘Vater Kopp’ Bio-, Biblio- und Psychographisches von und über Hermann Kopp (1817- 1892),” in Osiris, 5 (1938), 392-460, gives many personal details, largely drawyn from Kopp’s letters to Liebig.
Henry M. Leicester